Video circuit designs for providing synchronized video signals are useful with personal computers (PCs). Such designs place one image over another image on a PC display system and phase-lock multiple rasters (such as might be used in multiple display systems). The images can then be moved independently with movement commands to the video circuits. Further, a foreground image, such as an animation character surrounded by other background imagery, can be generated by giving portions of image around the animation character on the foreground image a transparency attribute, allowing the background imagery to be seen through the portions of the foreground image that have the transparency attribute. In the prior art, video circuit designs for providing synchronized video signals for the use of personal computers (PCs) in such applications are too large and expensive to be widely marketable to the public.
In the past, the method of painting top images on clear mylar or cellulose has been used and is widely accepted by animation artists. This is the same method that video game electronics companies use to electronically show small images known as sprites over large images. However, this has never been done with common video graphics adapter (VGA) PC-compatible computers. This overlaying of images is also known as color-keying, as a key color indicates transparency to the circuits. Color keying has been done before, but never on two or more raster images that had achieved the required synchronization and phase lock with a low cost circuit of the inventive type. Achieving synchronization of video raster scan circuits is easy and can even be done accidentally, if the same pixel clock is used for two or more taster scan circuits. However, phase lock is a concept that typically requires considerably circuitry.
The vast majority of video raster circuits that are available now cannot be synchronized. This is because the manufacturers of these circuits do not wish to add the expense of having all the horizontal pixel counters and vertical line counters with the feature of a zero reset. A zero reset feature is necessary to synchronize video raster circuits.
It is also desirable to have software that can operate effectively with multiple-monitor display systems. As operating systems and other portions of software on a PC change, the drivers necessary to correctly drive the display systems also change. It is, therefore, advantageous to have the driver software organized so that it can easily be changed in accord with the changes to the software that is involved in producing the information and images that are to be displayed.